Frequency selective device



y 29, 1952 R. CLCHEEK 2,605,396

FREQUENCY SELECTIVE DEVICE 7 Filed Jan. 21, 1949 2 SHEETSSHEET l 1 Fig. I. 1 Filter Uni: l I I i" -'i l Balanced l I T Modulator I M'xer 3 l I I9 I l r i 7 Phase I Phase I I I Splitter f J ph Splifler I Splitter Audio oiclllgior osc'fumor 9( j-.0u1pu1 lnqommg c+ m0 9 H l '7 f f 27 f0 Sugnql Phqse 23 5 Phase I I mbmer combiner I I I l5 I 7 I i Mixer l Balanced I- Mixer l I 5 Modulator I J l...... .l

7- T v J x Rejection Unit I Rejection u I V .B B B -B Li I [J I A A .A' v l F|g.2.. A Flg. 4. F|g.5.

Flg. 3.

3| 35 37 38 39 4| X F'l: F'fr' B d I 6 er er roc 29 Mixer Unit I Unit Filter Detector Ampllfler 5 Fig.6.

WITNESSES: INVENTOR Robert 0. Cheek. A MA- 8% 7 ATTORNEY July 29, '1952 R. c. CHEEK FREQUENCY SELECTIVE DEVICE 2 SHEETSSHEET 2 Filed Jan. 21, 1949 INVENTOR Robert C. Cheek.

l mw ATTORNEY Patented July 29', .1952

, UNITED "STATES PATENT ;,;0F-FICE FREQUENCY SELECTIVE Device Robert Cheek, Irwin, Pa., assignor to "Westinghouse Electric Corporation, East Pittsburgh,- 2a., 2i corporation of Pennsyivania H 7 Application January 21, 1949', Serial No; 71,825 i I v 11 Claims. 1

My invention relates to frequency-selective devices and it has particular relation to frequencyselective devices-havinga, uniform band-pass range with sharp cut o'ff characteristics.- v Frequency-selective devices such as filters are and capacitive reactances may be employed.

Although by employing multiple tuned circuits a desired bandpass characteristic may be approximated; it is impossible tqobtain therefrom the,

uniform transmission in the pass band and sharp cut-off at each end of the pass band-which is desired- .1. ;v s

In accordance with the invention a frequencyselective device is provi'edwh-ioh has sharp cut-'- off" characteristics. Ihe device comprises a filter unit which first converts an incoming; singlephase signal into a polyphase signal having a direction of phase 'rotation dependent on-the direction ofdeviatio'n of the. incomingisignal from a predetermined frequency, The-filter unit includes a phase combiner which is responsive to the phase rotation of the polyphase signal. Consequently the outputofthe phase combiner is responsive to those frequencies of the incoming signal which deviate -f i om the predetermined frequency in only one direction. I p The filter unit also includes a single-sideband modulator wherein a carrier having the predetermined frequency is modulated by the output of the phase-combiner to produce a sing-le-sideband signal which is similartothe incoming signal except for the rejection of all frequencies of the incoming signal which deviaterin a predetermined direction from the aforesaid predetermined frequency; t p

The filter unit embodying the invention, may be employedin' various circuits". .For' vthe purpose of illustration the filter unit will bedescribed as a component of a single-sidehand receiver and as a component of asuper heterodyne receiver. n I

Referring first to the single-sideband receiver, the output of the aforesaid single sideband'modulat'or may be supplied to asuitable demodulator which mixes the outpu't'fwith the output of an oscillator having theicarri'er' frequencyieniployed in thegeneration of the" singIe' s'ideBandsignaI at the transmitting station. A polypha'se signal 2- t V is obtained having a phase rotation" dependent on the direction of deviation of the frequency of the output of the single-sideban'd modulator from the carrier frequency. ,This 'polyphase signal is supplied to 'aphase combiner which is responsive to on'lyone direction of rotation of the polyphase signal. The, phaseco'mbiner is responsive to frequencies ofthe output of the single=sideband modulator which deviate only in a predetermineddirection from the carrier frequency. This direction is so selected that the output of the final phase combiner includes frequencies which lieonly within the desired pass band.

For the superheterodyne receiver twofilter unitsmay be employed in series,- each of which is similar to the previously described filter unit,

to pass only the band desired for the intermediate frequency section of the superheterodyne receiver. One of the filter-units is designed to reject ali frequeneies above the desired band, whereas the remaining filter unit isdesigned to reject all Jfrequenices of the incoming signal which are below the desired band. Y

It is therefore a Object of the invention to provide an improved frequency-selective device.

It is a further object of the invention to provide a frequencwselective device including a filter unit tor converting an incoming signal into a polyphase signal having adirection ofphase rotation dependent on the direetion of deviation of the frequency of the incoming signal from a predetermined frequency, and for modulating a carrier having. the predetermined frequency by a modulating quantity responsive to only one direction of "such phaserotation to produce a single-'sideband output. 7'

I-t'is also an object of" the invention to provide a sin gle-sideband receiver Whereina demodulator is ener ized from the filter unit-of the preceding paragraph, the demodulator comprising a mixer unit for converting the input thereto into a polyphas output havin 'phase rotation dependent on the direction of deviation of the input from a pre'deterinined frquency and the demodulator having a phase'conibinerresponsive to only one direction of rotation of such output;

It is a still'fu'rther object of the invention to provide a filter including two filter units 'in 'series, eachi similarjto' the filter unit described the pen-animate? paragrapnif f It is an additional object or, the invention to provide a .superhetferodyne receiver employing a filter similar-to that descf'ibedi'n thepreced-ifig paragraph. H' Q U.

Other objects of the invention will be apparent from the following description taken in conjunction with the accompanying drawings in which:

Figure 1' is a block diagram of a single-sideband receiver embodying the invention;

Figs. 2 through 5, inclusive, are vector diagrams showing vector relationships existing in the receiver of Fig. 1;

Fig. 6 is a block diagram of a superheterodyne receiver embodying the invention;

Fig. '7 is a schematic view of a circuit suitable for the single-sideband receiver of Fig. 1, and

Fig. 8 is a schematic view of a polyphase generator.

coming signal and of oscillator is selected for Referring to the drawings, Fig.'l shows'a filter unit I comprising a rejection unit I wherein an incoming signal is supplied in phase to two mixers 3 and 5. For the purpose'of discussiomit'i will be assumed that the incoming signal is received from a single-sideband transmitter wherein a carrier represented by its frequency fc is modulated by an audio signal, such as a voice signal represented by its frequency fa to produce a single-sideband signal. It will b assumed further that the signal transmitted is the upper sideband-which is a' function of the sum of the carrier and audio frequencies fc-I-fa. In the following discussion'the various signals and outputs will be represented by their frequencies.

Each mixer also is supplied from one phase of a polyphase 'gener'ator'or source of polyphase energy. In Fig. 1 the'polyphase generator is illustrated as'comprising an oscillator I which energizes a phasesplitter 9. For illustrative purposes the phase splitter 9 may split the output of the oscillator into two phase components differ'ing in phase by 90'to provide a source of twophase voltage. The'frequency of the oscillator is selected in accordance with th pass band desired. For present purposes, it will be assumed that the oscillator has a frequency fc-I-fma which is equaltothe 'sum of the-carrier frequency fc employedin generating-the incoming signal and the maximum frequencyjm of the audio signal fa- The outputs of the mixers 3 and 5 comprise two phase components of a polyphase output having a direction of phase rotation dependent on the direction of deviation of the frequency of the incoming signal from thefrequency of the oscillator I. Theoutputs of the mixers are supplied to a phase "combiner .I I. which is responsive to only one direction of such phase rotation.

The output of the phase combiner may be understood more clearly by reference to Figs. 2 to 5. These figures show the outputs of the mixers 3 and 5 by vectors A and B which ar assumed to have conventional counterclockwise vector rotation. It will be noted in Fig. 2 that the vectors illustrate a phase rotation wherein the vector A lags the vector B. Let it be assumed that the phase combiner II lags the vector A by 90 to the position illustrated by the vector A in Fig. 3. It will be noted thatthe vectors A and B now are in phase opposition and the output of the phase combiner II or resultant of the vectors A and B consequently is zero.

Let it be assumed nextthat the outputs of the mixers 3 and 5 hav the phase rotation illustrated in Fig. 4. In Fig. 4 the vector A leads the vector B by 90. Since the phase combiner is assumed tolag the vector A by 90 and to add the lagged vector to the vector B, the operation of the phase combiner is as illustrated in Fig. 5. In Fig. 5 the vector A has been lagged 90 to the position illustratedby the vector A. Since the the-output'of 'thephase combiner. This function has a frequency (ft-+fma) -(fc+fa) =fma-fa.

It will be noted that the output of the phase combiner isan audio signal which is a frequency tion of the direction of phase rotation to which the phase combiner I I responds, it"is possible to eliminate from the incomingsignal either all frequencies above the frequency-'fc-l-fm of the oscillator or all frequencies belowthat of the oscillator.

The selection of the-phase combiner may be altered by reversing one "of thephases supplied thereto orby changing the phase component lagged by the phase combiner;-For-example; let it be assumed that thef-phasecombiner II lags the vector Bof Fig. 2 by Thisj would bring the vector A into. phase agreement with the phase-shifted vector B and thephase combiner I I would have a substantial-output. When the same operation by the phase combiner is performed for the phase rotation of Fig. 4, the vectors are brought into phase -opposition and the phase combiner has substantially zero output. Consequently, by proper adjustment of .the rejection unit I frequencies of the incoming signal either above the oscillator frequencyfor below the oscillator frequency may; be rejected; as desired. As previously pointed OQt," i:b: Wi11. be assumed that the rejection unit I rejects all frequencies of the incoming signal higher than the'frequency fc-I-fma.

. for

The output of the rejection'unit I which is a function of ,the frequency jfllIr-fftl next is restored to a form substantially similar. to that of theincoming signal; Thisfoperation is effected by supplyingthe output'of the-phase combiner II to a single-sidebandQmodulator for modulating the outputof the oscillator 1. In Fig. l the single-sideband.niodulator comprises two balanced modulators I3 and' I5,i One phase component of the output; of the phase. splitter 9 is supplied to the balanced modulator I3, whereas the remaining phase co'mponentis supplied tothe balanced modulator I 5,. The output of the phase combiner 'issplitinto two phase components differing in phase .by 90? by means of a phasesplitter II. Qneof the phase components. of the phase splitter. I] ,is supplied to the balanced modulator I3, whereas the remaining phase component is supplied to the balanced modulators I5; e.

Since the modulators I3 andII5 are balanced, the carrier frequency supplied ,bythe. oscillator I is suppressed andlonly'the upper 'and'lower sidebands of the modulation products. appear atv the outputs. .of the. balanced. modulators. It

will be assumed-that theaoutputs of the balanced modulators-- are connected to supply the lower sideband which is a function of'the frequency (fc-l fma)(fnia ;fa) =fd+faw It will-be observed that the output QIP the -sig-nal-sideband modulatoris identical with the incoming signal 'eX- cept for the fact thatall frequencies, such as noise frequencies, higher inval-ue than the frequency-of the oscillator 1 are rejected.

It will-be recalled that the output of the filter unit may be-employed in various ways. I In Fig-.1 the filter unit isassociated with a rejection unit II or demodulator for demodulating the single-sideband signal.-' The rejection unit II includes a mi-xer lega mixer M, an oscillator 23',

a phase splitter-'25 and a phase combiner- 21*. The rejection unittII maybe similar to the rejection unit Iexceptfor the adjustment of the oscillator and the adjustment of the phase splitter or the phasecomb'iner.

The oscillator 23- supplies a carrier having a frequency fcequal to that ofthe carrier initially used in the production at-the transmitting station of thesingle-sideband signal which is to be received. Consequently, the outputs of the mixers. l9 and Zl comprise a polyphase output having. a direction of phase rotation. which is dependent on the direction of deviation :of the signal fc-I-fa from the frequency of the oscillator .23. The phase splitter and the phase combiner are adjusted. to. reject all frequencies of the incoming signal. which are lower in value than the frequency of the oscillator .23.; Consequently, the: output of the phase combiner 21 is equal to a function. of (fc+ja) --fc or the desired .audiosignalfa... The receiver of Fig. 1 consequentlypassesor.selects a frequency range represented bytherange of: the initial modulating signal fa. .All frequencies above. or below the desired pass band are rejected.

The filter .unit. of .Fig. lalso may be employed in. a superheterodyne receiver. in the manner illustrated in Fig. 6. The-incoming signal which may be derived from an antenna, 29 is supplied to a conventional first detector or mixer 3|, together with the outputof a. local. oscillator 33;. The output of the mixer includes. the desired intermediate frequency and may be passed to a band-pass intermediate-frequency filter be fore or after amplification. Such. a filter includes two units 3.5 and 31,,each of which is similar to the fi-lter unit of Fig. 1. However, the filterunit tfi of Fig. 6 has its oscillator adjusted for a frequency equalto the. maximum frequency to be transmitted thro ugh the passband and the phase-splitter and phase combiner of the filter unit 35 are connected to reject'all frequencies higherthan such oscillator frequency. The filter unit 31 has its oscillator adjusted to a frequency equal to the minimum frequency of the pass-band and the phase splitter and phase combiner are connected to rejectyall frequencies below the oscillator frequency of the filter unit Bl. As a result, thecombinei filter comprising the filter units 35 and 31 passes all frequencies within the desired pass-band and rejects all frequencies outside the desired passband. 'The filter unit 31 may be followed by a conventional second detector 39 and an amplifie r M for the purpose of supplying an input to asuitable translating device 43 such as a loud speaker. f

Ashereinafter set. forth, phase splitters and phasecombiners can'bedesigned which are effectiveovera'substantialfrequency range. With such splitters and combiners, the filter of Fig.

6 canbe designed to-pass uniformly any frequency within the desired pass band andto 'attenuate substantially any frequency fora "substantial rangeon each side of the pass band. If rejection of frequencies-for astill furtherrange is desiredya broad filter 38 -"may be provided.

This may be of conventional construction, and

filter units- 35 and 3?.

Suitable circuits for the single-sideband receiverof Fig. 1 areillustrated in Fig. '7. The incoming signal may be transmitted in any desired manner. For exampleit maybe transmitted through space However, it will be assumed that thejsignal is transmitted over suitable conductors such as two conductors of a three phase Bil-cycle power supply circuit 45. Theitwo conductors of thecircuit 45 between which the single-sideband signal appears may be coupled to the receiver through suitable coupling capacitors 4S and 54 and tuning inductance 0011s 59 and a transformer 52'. The secondary winding of the transformer 52'i's connected through a tuning capacitor 53 to the primary winding of a coupling transformer 54 having a tuning capacitor 55 across itssecondary'winding. I The receiver input, is supplied by the ---tr-a-nsformer- 54 to the two mixers-3- and 5. a

Any conventional mixer may be employed'for the'mixers 3 and 5. In the specific embodiment of Fig. 7, the mixer-3 comprises a pentagrid mixer tube 51 having five grids GI, G2, G3, G4 and G5 The tube also contains acathode-K and an. anode or-plate electrode 1?. Tubcs' of this type are well known in the art, one such type having the trade designation 6SA7.- The mixer 5-includesa mixer tube Fae-which is similar inconstruction to the tube 51'. 1

One terminal of the secondary winding of the coupling transformer 53 is connected to the grids G3 of thetubes 5 1 and 59. The; remaining terminal of the secondary winding is connected throughasuitable biasing battery 6|. to the cathode K of the tubes 51 and 59. The grids. G5. of the two tubes are suppressorgrids which are connected to the cathodes.. o'f the tubes. The screen grids G2 and G4 of the tube 5'! are connected. through a resistor63.to.the positive terminal of .aaplate voltage battery. 65. Thenegative terminal. of the battery .65 is. connected to the cathodes K of=the two tubes. In a similar manner, the screen grids Gland Gd of the tube 59 are connected througlra resistor 61 to. the positive terminal of the. battery: 65. A- by-pass capacitor Cl. is connected between; each cathode andv its associated screen grids. The plate electrode P ofthe tube 51 is connected to. the. positive terminal of the battery 65 through the primarywinding of an output transformer 69. In a similar-fashion the plate electrode P of the tube 59 is connected through the primary winding of an output transformer H to thepositive'terminal of thebattery65;

The'oscillator l" may be of any conventional construction. For power-line' carrier transmission, it conventional to' employ carrier frequenc' t e ordcrgo I D 09.13% kilocycl'esper second. "Stable electronic oscillators capable of oper ti n at uch fr ue, ci e wel known in ar .'..-wf. r 1.

j'llhe ut t oi t .qscil atoriliissupplied toth phase splitter; 9. In the embodiment-of 'Fig. '1

the phase splitter comprises a resistor 9a and a capacitor 913 which are connected in series across the outputof the oscillator 1. As these components. are designed; to .provide a resistive impedance ofthe resistor Sawhich is equal to the capacitive impedance of the; capacitor 9?) at the desiredfrequency ofthe oscillator I,;the voltages across the, resistor iandthe Capacitor will be equal in magnitude and will differ phase by 90. The voltage across the resistor-9a, is appliedbetween the grid, GI and the cathode K of the tube 51 through conductors 13 and 15. The voltage across the capacitor 9b is connected between the oathodeK and the grid GI of the tube 59 through the conductor 15 andaconductor 11. To equalizethe internal impedances of the output circuits of the-phase splitter, a capacitor 90 equal in capacitance to the capacitor9b may be inserted in the conductor 13. Also a resistor 9d equal in resistive impedance to the resistor 9a may be inserted in the conductor 11. A suitable biasing battery 19 maybe inserted between the cathodes K of the two tubes and the conductor 15.

It will be recalled that the outputs of the mixers 3 and 5 are applied to a phase combiner II. To this end, the secondary winding of the transformer 69 has connected thereacross a series circuit which includes a resistor 8|, an inductance coil 83and a capacitor 85. The coil 83 has inductively coupled thereto a coil 81. The coils 83 and 81 may be provided by sections of a single coil having a center tap 88 connected to one terminal of the capacitor-85. Acapacitor 89 is connected across the coils 83 and 81. The voltages across the capacitor 85 and coil 81 vary in magnitude inopposite directions as the frequency of the applied energization changes. Consequently, the voltage across the capacitor 85 and the coil 81 in series remains substantially constant and is displaced 90 from the voltage across the resistor 8! over a substantial range of frequency variation.

The voltage across the capacitor 85 and the coil 81 in series is applied through a resistor 9 I, across conductors 93 and 95. This voltage is added to the voltage across the secondary winding of the coupling transformer II. The two voltages will be in phase agreement or in phase opposition depending on the direction of deviation of the frequency of the incoming signal from that of the oscillator 1. One terminal of the secondary winding of the transformer H is connected to the coductor 93 and the remaining terminal is connected through a capacitor 91 and an inductance coil 99 to a conductor IUI. Consequently the output of the phase combiner II may be said to appear between the conductors 95 and IUI. A parallel resonant circuit comprising an inductance coil I03 and a capacitor I05 may be connected between the conductor 95 and a common connection between the secondary winding of the transformer 1| andthe capacitor 91, for the purpose of increasing the frequency range of the phase combiner II. p v i The capacitor 85 and the inductance introduced by the coil 83 across the secondary winding of the transformer 69 may be resonated to a frequency which is the geometric mean of the frequency range for which the phase combiner is designed. Similarly the parallel circuit representedflby the'ca'pacitor89 and the coils 83 and 81, the parallel circuit represented by the capacitor l ll5,and the coil I93 and the series circuit represented by the capacitors 91 and the coil 99, all may be resonated tothe aforesaid geometric main frequency. The resistor 8 Us useful in limiting current supplied to' the phase combiner and the resistor SI serves a similar purpose. A resistor I86 is connected between the conductors 93 and IM to serve as a load for the coupling transformer TI. The resistor I06, the capacitor 91 and the coil 99 preferably are proportioned in accordance with the impedance values of the resistor 8|, the capacitor 85 and the coil 83 to equalize the internal impedances of the two input circuits of the phase combiner. A phase combiner in a single sideband receiver is shown in the B. E. Lenehan patent application Serial No. 24,037, filed Apri1 29; 1948,-and assigned to the same assignee.

The phase splitter I1 follows the phase combiner 'II' and may be similar in construction except for thereversal of input and output circuits. The input circuit for the phase splitter I1 comprises in series, a' resistor I08, a capacitor I81, an inductance coil I09 and a current limiting resistor I I I. A coil H3 is inductively coupled to the coil I09 and the two coils may be constructed from a single winding having a center tap-H5. A capacitor I I1 is connected across the two coils I89 and H3. The voltage across the resistor I08 is substantially in quadrature with the voltage across the capacitor I81 and the coil H3 in series and equal thereto over a substantial range of variation of the frequency of the input to the phase splitter; Consequently, one phase output ofqthe phase splitter may be derived from the conductor IIII and a conductor II9, whereas the remaining phase may be derived'from the conductor H9 and a conductor I2I. In order to equalize the internal impedances of the output circuits for the phase splitter, the conductor IIII may be connected in series with a series circuit comprisinga coil I23 and a capacitor I25. The frequency'range of the phase splitter may be improved by connecting a circuit including a coil I21 and a'capacitor I29 in parallel between the conductor and the upper terminal of the capacitor I25 as viewed in Fig. 7. The internal impedances of the output circuits further may be improved or matched by including a resistor I28 in the conductor I2 I to compensate for the resistance introduced by the resistor I08. The phase splitter I1 is based on one illustrated in an article entitled A New Single-Sideband Carrier System For Power Lines by B. E. Lenehan which appears in vol. 66 of the American Institute of Electrical Engineers Transactions 1947 (paper 47-112). It will be understood further that the phase combiner II is substantially a reversal of the phase splitter I1. 7

The output of the phase splitter I1 is supplied to the single-sideband modulator which includes the two'balanced modulators I3 and I5. These modulators may be of any desired construction but in the embodiment illustrated in Fig. '7 they are illustrated as barrier-layer ring modulators.

The barrier-layer ring modulator I3 is energized in accordance with one phase component of the output of the oscillator 1 and one phase component of the output of the phase splitter I1. To this end a coupling transformer I3I has its primary winding connected across the conductors 13 and I5 for energization in accordance with the voltage across the resistors 9a. The secondary winding of the transformer 1*I3I is connected across ;o ne diagonal of the ringmodulator. The

output irom the ring rhodulato'rfis applied across the primary winding of an output transformer I33. Modulating voltage for the ri-ng modulator I3 is applied between center taps I3-Ia and I33d respectively on the secondarywinding of the coupling transformer I 3I and the primary winding of the output. transformer I33. These taps are connected respectively to the, conductor III and the conductor I I9for -energization in accordance with one phase component of the output of the phase splitter 'I 1-.

v i The ring modulator ;I5 is connected for energiza-tion -fromthe remaining phases of the phase splitter I1 and the oscillator "I in a similar manner. The primary winding of a coupling trans former I35 is connected between theconductors I5 and T! for energization in accordance with the voltage across the capacitor 5b. The I secondary winding of this transformer is connected across one diagonal-of the ring modulator I5. The output of the ring modulator is obtained through an output transformer 'l'iil having a primary winding connected across a second diagonal of the ring modulator. Center tape on the secondary winding of the transformer I35 and the primary winding of the transformer I31 are connected respectively to the capacitor I25'and to the conductor It!) for energization: in accordance with the voltage across theresistor I08; I

v Since the modulators I3 and I5 are balanced, the frequency of the oscillator I is suppressed. The outputs of :themodulators represent the sum and difference of two sidebands and these may be added or subtracted by connecting the secondary windings of the transformers I33 and I3! in series to provide either-the upper sideband or the lower sideband as desired. It will be assumed that thesecondary windings are connected to produce the upper sideband but it,

will be understood that by reversing the terminals of :the secondary windings of the output transformers I33: and I3I- the lower sideband may be produced. -A capacitor I38 may be connected across the, output terminals of the modu-. lator for the purpose bf tuning the output transformers. The single=sideband modulator of Fig. '7 is substantiallysimilar to that described in the aforesaid Lenehan article. v

The output of the single-sideband modulator is applied to the mixers I 9 and 2 i. These mixers may be similar .to the mixers 3 and 5 and similar reference characters identify similar parts. The

oscillator 23 and phase splitter 25 may be similar to the cscillatord'l and phase splitter in construction The only differences reside in the frecuesey to which the oscillator 23 is adjusted and the connections of the phase splitter 25 to the mixers I9 and 2I. It will be noted that the voltage across the capacitor 9b of the phase splitter 25 is connected across the grid GI and cathode K of thelower mixer 2I, whereas the capacitor 9b associated with the phase splitter 9 was connected across the corresponding elements of the upper mixer 3. The voltage across the resistor 9a of the phase splitter 25 is connected between the gridiGl and cathode K of the upper mixer Ill. The purpose of this reversal is to eliminate all frequencies from the incoming signal which are lower than the frequencs of the oscillator 23. The phase combiner 2! is similar to the phase combiner I I and is similarly connected to the associated mixers. The output of the phase combiner 21 is applied to a conventional amplifier MI and the output of the amplifier energizes afsiiitable translating device such as afloudjspeaker I43. I I

When a phase splitter 9 of the simple type illustrated in Fig. '7 is employed, the frequency of the associated oscillator cannot be altered appreciably without readj'usting the components of the phase splitter if optimum performance is desired. A phase splitter 9A permitting "substantial variation in frequency of the oscillator 1 without readjustment of the phase splitter is illustrated in Fig. 8. The output of the oscillator I in Fig. sis applied across a series circuit which includes a resistor 145, a capacitor M7 and an inductance coil I49. The inductance coil is mutually connected to an inductance coil 151. The coils may be constructed from a single winding having 'a cente'r tap connected to the, capacitor I141. .Inasmuch as the voltages across the coil I'5I f and capacitor Hi! vary oppositely in magnitude" in response to a change in frequency of the oscillator], the output voltage across the capacitor I41 and the coil 'I5I in series remains substantially constant over an appreciable range ofvariation in irequency of the oscillator 1. Moreover, this voltage is substntially in quadrature with thevol'tage across the resistor I 45. The voltage across the resistor I45 may be, applied acrossithe primary winding of an output transformer I53 which has its secondary winding connected between the conductors "I5fand71. The voltage across the coil I5I and the capacitor I4! in series is applied across the primary winding of an output transformer I55 which has its secondary winding connected through.a,i' eversing switch I51 to the conductors13 and I5. In order to equalize the internal impedances of the output circuits of the pnasesplitter, a resistor I59 may be included in the energizing circuit for the transformer I55 to compensate .for the resistor I45. For the same ,reason.an inductance coil I6! and a capacitor I63 may be included in the energizing circuit for. the transformer I53tocompensate for the capacitor I41 and the inductance introduced by the coil I 5| in the energizing circuit for the transformer, I55. A I i v v v The capacitord lfl and the inductance'intro duced. by the coil I49 across the oscillator I may be resonated to the geometric I means of the range of frequencies over. which the oscillator I is to be adjusted. Similarly the series resonant circuit comprising the coil I61 and the capacitor I53'n1ay be resonated to the same geometric means frequency. If the phase splitter of Fi {3 is employed in place of thatillustrated in Fig. 7 the frequency of the oscillator I may be adjusted over' an appreciable range without disturbing the phase splitter. The provision of the reversing switch 'I5I permits a'reversal-of the phase rotation 'o'i'th'e outputs of the mixers 3 and 5 without necessitating a reconnection of the phase splitter. A phase Splitter Similar to that illustrated in Fig. 8 also may be employed as the phase splitter 25 of Figs. 1 and 7.

Although the invention has beendescribed with reference to certain specific embodiments therecf numerous modificatiohsfialling within the spirit and scope of the invention are possible.

I claim as my invention:

1. In electrical device for passing frequencies within a band having a predetermined limiting frequency, a filter unit comprising a source of alternating voltage having said limiting frequency, a phase-splitting mixer for combining 11- an incoming signal with the output of the source to provide a plurality of signal components differing in phase, said filter unit including a phase combiner for combining the'signal components to provide a resultant signal, a modulator for modulating the first resultant signal and the output of the source to produce a, pair of modulator outputs having frequencies which include a function of the sum and a function of the difference of the frequencies of the resultant signal and of the source, and means for adding the modulator outputs to provide a second resultant output which comprises. only one of said functions.

2. A device as. defined in claim 1 in combination witha second source of alternating voltage having a second limiting frequency, the band being between said two, limiting frequencies, a phase-splitting mixer for combining the second resultant output with the output of the second source of alternating voltage to provide a plurality of second signal components differing in phase, and a phase combiner for combining the second signal components to' provide a third resultant signal.

3. A device as claimed in claim 2 in combination with a single-sideband modulator for modulating thev lastnamed'third resultant signal and the output of the second source and selecting one of the sidebands of such modulation.

4. In an electrical device for passing frequencies within a band having a predetermined limiting frequency, a polyphase source of alternating voltage, a mixer for mixing an incoming signal with each of a plurality of carrier phase components derived from thesource to provide a plurality of signal components differing in phase, a phase combiner for combining the signal components to provide a first resultant single-phase signal, and a single-sideband modulator for mode ulating the source by the. single-phase signal to produce a single-sideband signal. 7

5. A device as defined in claim 4 in combination with a second source of alternating voltage having a second limiting frequency, the band being between the two limiting frequencies, a phase splittingmixer for combining the single sideband signal with the output of the second source to provide a plurality of second signal components differing inphase, and a phase combiner for combining the secondsignal components to provide a second resultant signal.

6. A device as claimed in claim 5 in combina-j tion with a single-sideband modulator for modulating the second resultant signal and the output of the second source and selecting one of the sidebands of such modulator.

7. A device as claimed in claim 4 in combination with a filter having a pass band substantially larger than the first-named band.

8. In an electrical device for passing frequencies within a band having a predetermined limiting frequency, a two-phase voltage source having said limiting frequency, a mixer for mixing an incoming signal with a first phase of the source to provide a first signal componentya mixer for mixingthe incoming signal with a second phase of the source to provide a second signal component} said signal components comprising a two-phase output having a phase rotation direction, dependent 'on the direction of deviation of the frequency of the incoming signal from the frequency of the source, a phase combiner for rotating one of the signal components into phase or phase opposition relative to the other of the signal components depending on the direction of said phase rotation and adding the signal components after such rotation by the combiner to provide a first resultant signal substantially responsive to an incoming signal having a frequency located on only one side of the limiting frequency, an a 'single-sideband modulator for modulating the output of the source by the resultant signal to'produce a single-sideband signal. 1

9. A device as defined in 'claim 8 in combination with asecond tvio-phasevoltage source having a second limiting'frequency, the band being between the two limiting "frequencies, a mixer for mixing the single-sideband signal with a first phase of the s'ource'to' provide a first output signal, a mixer for mixing the single-sideband signal with a second phase of the source to provide a second output signal, said output signals comprising a two-phase output having a phase rotation direction dependent on the direction of deviation of the single-sideband signal frequency from the frequency of the second two-phase voltage source, and a phase combiner for rotating one of the o'utputsignals into phase or phase opposition relative to the other of the signal components depending on the direction of said lastmentioned phase rotations and adding the output signals after such phase rotation by the last-mentioned phase'combiner to provide a second resultant output signal.

10. A device as defined in claim 9 combination with a single-sideband modulator for modulating the output of the second voltage source by the second resultant output signal and selecting one of the'sidebands of such modulation.

11. A device as'defined in claim 9 wherein at least one of the sources is' adjustable for producing outputs of various frequencies.

ROBERT C. CHEEK: f

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Practical Single Sideband Reception, QST, July 1948, pages 11 to 15. 

